Programs like Audacity let you create a "noise profile" which is then used to remove noise on the recording. If you can find a few seconds where the only sound is the chalk-board noise, then you can try that.
IME, noise reduction works better in multiple passes than trying to catch it all in one pass.
I know it won't help what you've already recorded, but I think the real solution here is to think about where and how you position and configure the mic.
That said, +1 for Audacity to clean up the existing recordings.
The most annoying part of the chalk is probably the higher frequencies (squeaking), so a simple EQ pass might be able to make the recording more tolerable. Audacity has a few plugins that do EQ. A bit of trial and error and you should be able to quiet the highest frequencies while leaving the voice audible.
You could always use a sound-editing program like Audacity (free, open source) and clip them out.
I'm not sure what to do if the chalk noises happened while the instructor was talking, though.
